The movable contact of an electromagnetically actuated circuit controlling device such as a DC contactor is pulled at high velocity into engagement with a rigid stationary contact when the armature is attracted to the magnetic core, and the movable contact tends to rebound when it strikes the rigid stationary contact. The consequent bouncing of the parts when the movable contact is stopped suddenly causes arcing which produces wear and erosion of the contacts and renders the circuit controlling action of the device uncertain. The erosion of the contacts due to such arcing results in a decrease in contact pressure with a consequent increase in contact resistance and in heating of the contacts and may eventually cause contact welding. I have also found that accumulation of falling dirt and dust in the armature pivot area may mechanically jam the armature and cause its pivoted end to move in a direction away from the yoke rather than pivoting in line engagement with the yoke. Further, the movable contact carrier bar of certain prior art contactors was permitted to tilt about its longitudinal axis relative to the armature for the purpose of providing wiping action between the contacts, and it has been found that such tilting motion of the movable contact carrier bar resulted in "rotational bounce" when the movable contact struck the rigid stationary contact which contributed to contact-bounce, and thus to contact welding, and also caused a tapered wear pattern on the contacts which prevented use of full volume of the silver contact material.
In order to reduce rebound of the movable parts, it is known to provide a pre-loaded contact pressure spring which biases the movable contact carrier toward the armature so they move as a unit until the movable contact engages the stationary contact, after which the armature continues to travel so that the force of the pre-loaded contact pressure spring is applied against the movable contact carrier to effect high contact pressure and thus reduce contact bounce. Other known arrangements utilize an inertia weight which additionally compresses the contact pressure spring when the armature closes in an attempt to prevent the contacts from bouncing. As far as I am aware, such prior art arrangements do not provide maximum magnetic efficiency, i.e., maximum pull per ampere, and have relatively low contact pressure spring force with the result that they are relatively ineffective in preventing contact bounce with consequent contact welding when high magnitudes of current flow through the contacts. Specifically, I have found that typical prior art contactors have magnetic circuits with relatively low reluctance in the open armature position but relatively high reluctance in the magnet gap region adjacent the closed armature position and, further, that the magnet gap in such prior art contactors at the point of contact engagement is greater than the magnet gap at the knee of the generally hyperbolic magnetic pull characteristic. As a result the characteristic has a relatively gradual slope above the knee; the flux density (and thus magnetic pull per ampere) is limited in such prior art contactors by the relatively high reluctance in this magnetic gap region above the knee; the combined force of return spring and contact pressure spring in the closed armature position tending to overcome the residual mmf and accelerate the armature and contacts to open position is similarly limited; and also the force of the pre-loaded contact pressure spring (which reduces contact bounce) must be less than the magnetic pull at the knee and also less than that of the return spring because of the magnet gap at the point of contact engagement.